1. Field of the Invention
This invention relates to a process for producing an imaged article and in particular it relates to such a process whereby sensitizing dye is bleached by the simultaneous exposure to heat and radiation.
2. Background of the Invention
It is well known in the imaging arts that photolyzable organic halogen compounds, such as diaryliodonium salts and triarylsulfonium salts (e.g., "onium" salts) and halogenated triazines, can be sensitized with a variety of dyes (e.g., diphenylmethane, xanthene, aminoketones, etc.) in imaging systems involving polymerization of negative-acting, free-radically polymerizable monomers. See, for example, U.S. Pat. No. 3,729,313. Additionally, such photosensitive compositions can be employed in positive-acting imaging systems, whereby, for example, a polymer which has been rendered insoluble by the presence of a solubility inhibitor is solubilized upon the exposure of a combination of sensitizing dye and diaryliodonium salt or halogenated triazine to radiation.
Such sensitizers also sometimes function as anti-halation or acutance dyes. As is known in the imaging arts, such dyes serve to absorb incidental or scattered light in light-sensitive imaging layers. By absorbing the scattered light, a reduction in image sharpness is avoided.
Regardless of the function of the dye, it will often be necessary to bleach any remaining dye after an image is formed because residual dye color can be objectionable. Dye bleaching is well known in the art and typically involves the use of heat or light or combinations thereof.
U.S. Pat. No. 3,658,521 discloses the simultaneous use of light and heat to bleach 1-aminopyridinium dyes which are useful for sensitizing photoconductive compositions used in electrophotographic layers. Bleaching of the dye eliminatels undesirable color imparted to background areas of an image-bearing element, thereby increasing the visual contrast of the bleached element. The bleaching of dyes in conjunction with either aromatic iodonium or halogenated triazine compounds is not disclosed.
U.S. Pat. No. 3,788,849 discloses a process for forming visible images by exposing a thin film of a dye (e.g., diphenylmethane, triarylmethane, acridine, and xanthene dyes) having an absorption in the visible wavelength region to light which will be absorbed by the dye and heating the thin film simultaneously, or immediately after the exposure, whereby the dye is faded in the areas which have not been exposed to light. The use of aromatic iodonium salts or halogenated triazine compounds are not disclosed.
U.S. Pat. No. 4,548,896 discloses an imagewise bleachable composition containing a dye in reactive association with a mesoionic compound. The composition may be bleached upon exposure to radiation having a wavelength in the range of about 200 to 1100 nm and/or upon heating to at least 70.degree. C. Aromatic iodonium salts and halogenated triazines are not disclosed.
U.S. Pat. No. 4,594,312 discloses heat-bleachable polymethine dyes used as acutance/antihalation dyes in dry silver materials. The use of aromatic iodonium salts or halogenated triazine photoinitiators is not disclosed.
U.S. Pat. No. 4,632,895 discloses the formation of a positive dye image by bleaching of a dye with iodonium ion. The iodonium ion is exposed to radiation in order to initiate the bleaching process.
U.S. Pat. No. 4,701,402 discloses a radiation-sensitive element capable of recording an image upon image-wise exposure to radiation of selected wavelength. The element contains an effective amount of a bleachable dye in reactive association with an iodonium ion as the image-forming components. Suitable dyes include polyroethine dyes having an oxidation potential between 0 and .+-.1 volt.
European Patent Office Publication No. 424,124 discloses a positive-acting photoresist composition containing: (i) an alkali-soluble binder having a plurality of phenol groups; (ii) a diaryl iodonium salt; and (iii) an acid cleavable compound. A sensitizer for the diaryl iodonium salt is also disclosed.
Light-sensitive articles have been employed in the preparation of overlay or surprint color proofing sheets, which are used to verify the accuracy, with respect to color, of separation films used in the preparation of printing plates. A single-sheet, four color proof (a surprint) must duplicate with great accuracy the colored image and background to be achieved on a printing press using printing plates made with the same separation films. Such articles are well known in the graphic arts and typically comprise a photoreaction initiator, a photoreactive (polymerizable or depolymerizable) compound, a photoreaction initiator, a sensitizing dye for the photoreaction initiator, and a pigment. Processes and methods of color proofing are described in M. H. Bruno, Principles Of Color Proofing: Gama Communications; Salem, N.H.; 1986.
The purpose of the photoreaction initiator (i.e., photoinitiator) is to generate free radicals upon imagewise exposure of the photoreactive layer to actinic radiation to initiate polymerization (or depolymeization). The sensitizing dye serves to sensitize the photoinitiator to the radiation of the light sources used. The pigment is a colored material that very closely matches the color of the inks that will be used in printing. Thus, the final color of the pigment is the desired final color of the proof.
One common method of developing such an image uses the solubility difference created between the photoreacted and unphotoreacted material. For example, the photoreacted material may be less soluble in the exposed regions and more soluble in the unexposed regions. Removal of the soluble material with an appropriate solvent results in an image of photoreacted material in the exposed regions.
Another common method of developing such an image uses a laminate construction. The material in the exposed region preferentially adheres to one of the layers in the laminate and can be separated from the material in the unexposed region by peeling the laminate apart. A general review of photopolymerization and development techniques is described in C. G. Roffey, Photopolymerization of Surface Coatings; John Wiley & Sons; New York, N.Y.; Chapters 4 and 6, 1982.
Whenever a layer of a photoreactive material that is a component of a proofing sheet construction comes into contact with a layer of resin (such as is present in a laminate peel-apart construction) it is possible for one or more of the colored components of the photoreactive layer, in addition to the pigment, to migrate into the layer of the laminate and cause discoloration of the transferred pigment. An "off-color" proof then results.
U.S. Pat. No. 4,933,452 discloses triazine derivatives of polymeric or oligomeric compounds having a reduced tendancy for migration. However, these triazine compounds require a sensitizing dye be incorporated into the photoreactive construction in order for the photoinitiator to be responsive to the radiation of the light sources used in graphic arts applications. This sensitizing dye can migrate into the laminating layer and cause discoloration of the subsequent proof. Unless this sensitizing dye can be bleached after imaging or after transfer to the proof, an "off-color" proof can result.
In a typical imaging construction employing sensitizing dyes for iodonium salts and halogenated triazines, generally at least about ten percent of the actinic light to which the construction is exposed will be absorbed by the dye in the wavelength range of about 400 to 600 nm. In such constructions one would expect the need to use an exposure fluence (i.e., average exposure of radiant energy per unit area expressed in Joules/cm.sup.2) of visible light energy on the order of 20 to 200 J/cm.sup.2 to render a bleachable dye colorless or subject to no further color change in a commercially practical period of time. However, in order to prevent thermal damage to substrates such as paper or films typically used in photographic constructions, it is generally necessary to use fluence rates of less than about 3 J/cm.sup.2 /min. Thus, typical bleach exposures will require from about 5 to about 60 minutes of exposure. This is impractical for most product applications. It would be desirable to have an imaging process whereby effective image formation occurs, yet relatively low radiation fluence and dye bleach exposure rates are employed.